From what I've seen, there's no real way to control the *synced* rate on one of Massive's LFO's with a macro, simply because the knob disappears. You may just have to automate the rate separately. Once the sync tool is activated within the LFO, the designated automation control becomes: "MODULATOR5-DEN" (Assuming you are using Modulator #5). I'm using Logic so not sure if this will remain to have the same label in other DAWs.

Here are the rates by number amount that I've tested when controlling this automation, just incase you need:

.13 = 1/2
.25 = 1/4
.38 = 1/8
.50 = 1/12
.63 = 1/16
.75 = 1/24
.88 = 1/32

For example, if your automation line is anywhere between .13 and .24, it will have a 1/2 rate. At .25 it becomes 1/4, and so on for all intervals.

Sorry if confusing, hope this helps you make some cool LFO rhythms